MacBook unable to send or upload files along with other issues after Sequoia OS update

Mac can't send/upload/put out files that aren't super-tiny at all after OS update from Big Sur to Sequoia, even after a full Recovery Mode re-installation of OS

SO! Four years ago I tried to update my MacBook Pro to Big Sur and it bricked the whole system. 'Not enough storage' to download and uncompress the new OS, but the old OS was already deleted. So, no OS and no way to install an OS. Had to get the thing hard-wiped, buy a second one in the interim for work, and lost all my data that wasn't on any number of clouds. Let's just say I have an aversion to OS updates now.

Anyway my new MacBook and my Mac mini have been on Big Sur ever since. I started getting end-of-service notices from apps and decided it was time to bite the bullet and updated to the current version of Sequoia (the one previous to the one that JUST came out yesterday). I was scared for my Mac mini based on storage differences, but that one has been updated just fine and has so far not had any issues! Hooray! Success!

My MacBook on the other hand… Halfway through the installation the progress bar stopped for about an hour. Following online advice (and maybe a little anxiety) I hard-shutdown the machine and started it up, and the new OS seemed to be just fine. Just a stall, rebooting finished it off, etc. Or so I thought.

One by one I started finding problems that all seem to be tied with the MacBook trying to upload, sync, send, or otherwise 'put out' any reasonably sized files. Replying to a long email thread wouldn't work on any outgoing mail servers on Mail, even if it worked from any other device and plain text emails send just fine (outgoing server "null", if any message is given), and trying to send an email with any attachments does the same. Then Gyazo won't be able to upload any screenshots or recordings bigger than a few pixels ("broken pipe" or "SSL_read: sslv3 alert bad record mac") Then Google Drive and Dropbox won't sync files when they've been added straight into the folders. Then I can't upload a PDF to an external website. Then I can't send images on Discord. I've even just tried a speed test and while download is over 600mbps (which is usually symmetrical for both download AND upload), the upload test doesn't even start.

It's like the outgoing information flow on my MacBook has suddenly been tightened to a trickle. I can send out or upload tiny files, plain text messages, and download anything and everything thus far just fine. It literally just seems to be any time I try to put out anything larger than a megabyte at a time, and it only affects this device, no others. I've made a dummy admin user to test it there, same deal. I was advised by a lovely Apple call handler to reinstall my entire OS after holding the power button down for 15 seconds, which worked fine and didn't even stall this time. Still no dice.

I don't know what else to do. If overhauling the entire OS hasn't fixed the problem I don't know what this could be.

[Re-Titled by Moderator]